The FBI and U.S. Justice Department are investigating possible U.S. ties to alleged corruption involving the former president of Ukraine, including the work of firms headed by political operatives Paul Manafort and Tony Podesta, CNN reported on Friday, citing multiple U.S. law enforcement officials.

The broad-based investigation was looking into whether U.S. companies and the financial system were used to enable corruption by the party of former pro-Russian Ukrainian president Viktor Yanukovych, CNN said.

A person who answered a telephone number for Manafort said Manafort was not available for comment. The person, who said he was an associate of Manafort and who gave his name only as David, referred queries to a lawyer in Washington, who did not immediately respond to a phone call and an email.

In response to a report in the New York Times on Monday, Manafort denied any impropriety in a statement. "I have never received a single 'off-the-books cash payment' as falsely 'reported' by The New York Times, nor have I ever done work for the governments of Ukraine or Russia," he said.

The New York Times reported that he had received cash payments worth more than $12 million over five years that were itemized on secret ledgers belonging to Yanukovich's Party of Regions.

Manafort, who resigned as chairman of Republican presidential nominee Donald Trump's campaign on Friday, had not been the focus of the probe, CNN said, citing the officials.

The probe was looking at the work of other firms linked to the former Ukrainian government, including the Podesta Group, a lobbying and public relations company headed by Tony Podesta, whose brother John Podesta is chairman of the campaign to elect Democratic Party presidential nominee Hillary Clinton.

Manafort's attorney Richard Hibey did not immediately respond to a request for comment.

The FBI declined to comment, CNN reported.

The Podesta Group has hired an independent legal firm to investigate whether it had been misled by the Centre for a Modern Ukraine, a not-for-profit group linked to the ousted Ukrainian government, a spokeswoman for the group said in a statement.

The Justice Department, asked to comment on the report, said it remained "committed to helping recover stolen assets on behalf of the people of Ukraine."

Investigators in Ukraine have said Yanukovych and his party engaged in widespread corruption. He fled to Russia following a popular uprising in 2014.